Shale distillation



March 13, 1956 H, z, MAR-HN ETAI.

SHALE DISTILLATION Filed OCT.. 3l, 1951 `Lijiconuev' .mm-tin. :bm/encore cmrren. .Lew-.5 Jr'. [7W Ottorne mm .mit mm\ o e f .L m @m m@ xr Nm 1| rlllhvh |v m- Nv mm 2 +o# .6mm m0 mw @L mg me @e Hm. .E uw lv L AI w mjww: mm, NL... Il Whm/ Q MMFPMIMTQ m m AI lwfwdwlllmwlhmhl H km? b# nl mdxllll MPJ Q T wf: mmv

United States Patent O SHALE DIsTlLLA-'rIoN Homer Z. Martin, Cranford, and Warren K. Lewis, Jr.,

Elizabeth, N. J.,`assignors to Esso Research and Engineering Company, a corporation of Delaware Application October 31, 11951, Serial No. 254,162

7 Claims. (Cl. 2112-14) The present invention relates to the distillation of oil shale. More particularly, the invention involves an improved method of distilling oil `shale affording substantial economies in the grinding and preheating stages of the process. In brief compass, the invention provides for preheating a 4substantial proportion of the raw shale in the form of relatively coarse lumps by a direct countercurrent heat exchange with hot product vapors and gases while preheating the remaining portion of the raw shale in the form of` a nely ground material by indirect heat exchangewith hot spent shale and subjecting the total char-ge so preheated to distillation in the form of a dense turbulent mass of subdivided solids tluidized by upowing gases and vapors.

Prior to the present invention, it has been proposed to distill oil shale in the form of a dense, turbulent, uidized mass consisting of particles up to 1/2 inch in diameter. The heat required for the distillation is supplied by burning combustible shale constituents either in the distillation zone itself or in a separate combustion zone from which hot solids heated by the combustion are returned to the distillation zone.

In processes of this and similar types, the major heat load is that required for raising the cold shale to retorting temperatures in excess of about 800 F. If it is attempted to accomplish this by feeding the cold shale directly into the distillation zone so as to heat it up to distillation temperature in contact with the shale undergoing distillation, excessive amounts of heat must be generated by combustion to maintain the total charge at distillation temperature. This involves serious losses of volatile shale constituents which are burned when the combustion is carried out in the distillation zone itself. Excessive temperatures, undesirably high solids circulation rates and/or excessive quantities of air are required when a` separate combustion zone is used. Ecient utilization of the sensible heat of the distillation products and residue in preheating the shale is, therefore, of considerable importance for the economics of the process.

Heretofore, various proposals havev been made to accomplish this by a direct or indirect heat exchange between cold shale and hot gasiform effluent of the distillation zone. For example, in accordance with one of these proposals shale of iluidizable particle'size, i. e., not exceeding about 1/zv inch in diameter, is fluidized in a preheating zone by an upllowing suspension of spent shale fines in hot distillation vapors and gases at ternperatures above the dew point of.\ the vapors. When so operating, the temperatures of the preheated shale and the cooled product vapors must be about the same as a result of the uniformity of temperature prevailing throughout uidized beds. cooled merely to a level which requires extensive and expensive additional coolingv for condensation. Fluid operation of the heat exchange zone also requires relatively ne grinding of the shale to a uidizable size.

The product vapors are ice Z This type of grinding is extremely expensive, particularly with respect to power requirements.

Another prior art proposal involves cooling and condensing the product vapors in direct heat exchange with cold fresh shale in a transfer line. The product vapors and the fresh shale are further heat-exchanged in a fluidtype scrubbing vessel wherein entrained spent shale nes are scrubbed from the gasiform products by means of product condensate. Again uid operation of the` heatexchanging scrubber requires fine grinding of the fresh shale and extensive additional cooling for product recovery. i

Prior processes involving heat exchange between cold fresh shale and hot spent and/or burned shale likewise involve fluid operation in the heat exchange zone and, therefore, similar dilhculties. The present invention avoids these difficulties. A

It is, therefore, the principal object of the invention to provide improved means of exchanging heat between fresh cold shale and the yoverhead and residue from a fluid shale distillation process. Other objects and advantages will appear from the subsequent description of the invention, wherein reference will be made to the accompanying drawing, the single figure of which is a schematical illustration of a system suitable to carry out a preferred embodiment of the invention.

In accordance with the present invention, a substatitial portion, preferably a major portion, of the fresh cold oil shale charge is coarsely crushed to lumps rang ing in size from about 1/2-5 inches, preferably `mostly about 1-3 inches in diameter. The remainderof' the shale is treated as will appear hereinafter.

The coarse fraction representing the bulk of the charge, say about 50-80 weight percent thereof, preferably about 60,- weight percent, is passed in the form of` a settled, essentially non-turbulent moving bed downwardly through a preheating zone in direct countercurrent heat exchange with the hot gasiform overhead from` a fluidtype shale distillation zone. Since the shale is not fluidized in this preheating zone, no line grinding is required. The preheated shale leaving the preheating zone is fed fluid-type distillation zone.

Upon reaching distillation temperatures of about 800-l200 F. in the distillation zone, the coarse shale tends to disintegrate rapidly into particles which are fine enough to iluidize readily at superficial linear'gas velocities of about 0.3-5 feet per second. If desired, coarse preheated shale lumpsl which fail to disintegrate in this manner, may be subjected to disintegration by means of a high-velocity gas jet either in the transfer line leading from the preheating zone to the distillation zone or in any portion of the distillation zone wherein such large lumps tend to accumulate. This type ofdisintegration is adequate because the disintegrationv resistance of the shale has been greatly reducedA by the preheating treatment.

As a result of the direct countercurrent contact in. the heat exchange zone, the overhead from, the distillation zone may be cooled to temperatures closely approaching that of the incoming raw shale, thusy greatly reducing, oreven completely eliminating, the need and expense for indirect cooling surface and cooling water.

Simultaneously, the raw shale may be preheatedjtorela-v tively very high temperatures which may even closely approach distillation temperature itself. The shale pre,- heating temperature depends largely on the time of con.- tact between shale and distillation overhead in the preheating zone. Assuming distillation temperatures of about 800-1000 F., shale preheating temperatures of'about 50W-900 F. may be obtained at shale residence times. ofA about 5-l00minutes in the preheating zone.

In normal operation, the raw shale has a higher heat content than that of the gasiforrn overhead from the distillation zone. Therefore, the heat content of this overhead is insucient to preheat the total raw shale charge of the distillation zone to a desirably high temperature level of about 500-900 F. For this reason, only a portion, though preferably a major portion, of the shale is preheated in this manner.

The remainder of the total shale charge is preheated in heat exchange with spent hot shale. This is done by indirect heat exchange and preferably by means of a ypebble heater system wherein the lluidized raw shale is -heated by direct contact with hot pebbles which, in turn, .have been heated by direct contact with a fluidized mass of hot spent shale. For this purpose, the raw shale 'must -be ground to a uidizable size not exceeding about 1A :inch particle diameter with particle sizes of less than 200 microns predominating. Shale preheating temperatures of about 600 800 F. may be readily attained in this manner at spent shale temperatures of about 80021000"v F. Higher preheating temperatures may be reached when hot burned shale from a separate combustion zone operated substantially above distillation temperature is used to heat the heat exchange pebbles. The minor proportion of raw shale so preheated is likewise passed on to the distillation zone proper.

Any conventional fluid-type operation may be employed for the distillation stage itself. The heat may be supplied by carrying out a limited combustion within the distillation zone. For this purpose, a free oxygen-containing gas is used to uidize the shale in the distillation zone. Or a two-vessel type of operation may be used wherein spent shale is circulated between the distillation zone and a combustion zone operated at a substantially higher ternperature, all the heat required for distillation being supplied in the form of sensible heat o f burned shale. Systems of this type are well known in the art as exemplified by such patents as U. S. 2,396,036; U. S. 2,471,119; U. S. 2,480,670; etc.

It may be desirable to minimize solids fines entrainment in the overhead gases and vapors from the distillation zone. This may be accomplished by employing suitable gas-solids separation means or by cooling the overhead in the top of the distillation Zone just below the dew point of the vapors, say to about 700-900 F., so as to exert a scrubbing action on the overhead, or by a suitable combination of such means. Most of the fines carried overhead from the distillation zone into the shale-overhead heat exchanging zone will be retained therein as a result of the frequent and rapid changes in the direction of gas flow in the densely packed moving shale bed and due to the scrubbing action of liquid condensing thereon.

Shale oil condensation in the overhead-shale exchanger is unavoidable at the conditions of the invention. It is generally undesirable for this condensed oil, particularly constituents boiling within and below the gas oil range to be returned to the distillation zone together with the preheated raw shale. This can be prevented either by removing the condensate in suitably arranged trays or by employing gas velocities sufficiently high to entrain and carry the condensate out the top of the heat exchange zone. Since the highest boiling constituents will condense first in the lower portions of the heat exchange Zone and a recracking of these constituents in the distillation zone may be desirable, the gas velocity may be maintained at a liquid entrainment level only in the upper portions of the heat exchange zone. This may be accomplished by adequately reducing the diameter of this Zone in its upper portions.

When operating in accordance with the invention, the oil shale may be preheated to high temperatures in inexpensive equipment, whereby oil losses by combustion as well as air compression requirements and retort size are reduced by economical means. The invention also reduces grinding requirements to a fraction of those of conventional uid operation and permits cooling of the distillation overhead to substantially ambient temperature without the need of expensive cooling equipment and cooling water which is at a premium in most shale mining areas.

Having set forth its objects and general nature, the invention will be best understood from the more detailed description hereinafter read with reference to the drawmg- I Referring now 'to the drawing, the ysystem illustrated therein essentially comprises a heat exchanger 7, a`dstillation retort 15 'and a pebble heater system 20, 28, 75, the functions and coaction of which will be forthwith described. p

, In operation, raw oil shale crushed to lumps of up to 'about 2 inches diameter is supplied to the system via line l. About 20-40 weight per cent of the shale is further ground in a conventional grinder 3 to a fluidizable particle size not exceeding about 1A inch in diameter, about 60-90 weight per cent of this material passing through a 50 mesh screen. The disposition of this ground. material will be described hereinafter. The major portion, i. e., about 60-80 weight per cent, of the coarse crushed shale is passed by means of any conventional conveying system indicated by line 5 to the top of heat exchanger 7. This exchanger may have the form of a simple substantially cylindrical vertical drum having a heightzdiameter ratio of about 2-5 1. It may be provided with gate valvetype feed mechanism 9 and solids discharge means of the screw conveyor or any similar type arranged in the bottom portion as indicated at 11.

Hot overhead vapors and gases formed in retort 15 as will appear hereinafter are supplied through line 13 to the bottom of exchanger 7 at a temperature slightly below distillation temperature, say at about 750-1l50 F. The vapors and gases flow upwardly through exchanger 7 at a linear velocity of about 4-30 feet per second in direct countercurrent contact with the downflowing shale. The feeding and discharge means of exchanger 7 are so operated that a dense settled bed B of coarse shale is formed which moves through exchanger 7 at a rate providing for a shale hold-up or residence time in exchanger 7 of, say, about 5-100 minutes depending on the prevailing temperature conditions. When so operating, the coarse shale may be withdrawn at a temperature of about 500- 1000 F. and passed at this temperature by means of conveyor 11 and line 14 to an intermediate portion of restort 15, to be treated therein as will appear more clearly hereinafter.

Returning now to grinder 3, the portion of the shale charge nely ground therein is supplied through line 17 to a lower portion of shale preheater 20 which may have the form of a conventional type fluid-solids treating vessel. The shale may be fed through line 1'7 by means of a d screw conveyor, aerated standpipe, or any other suitable solids feeding means known in the art of fluid solids handling. Preheater 20 contains a relatively dense mass of heat-carrying pebbles having a particle size of about 5-30 mesh, continuously supplied to the top of preheater 20 from pebble heater 75 at a temperature of about 600-l000 F., as will appear more clearly hereinafter. The pebbles may consist of ceramics, aluminum, stainless steel or other high heat content material. They may be withdrawn from the bottom of preheator 20 through line V23 at a temperature of about l50300 F. A uidizing gas, such as product tail gas, flue gas, steam, etc. 1s supplied via line 25 and a distributing device, such as a perforated cone 27 to a level below the point of entry of the fresh shale.

The superficial linear velocity of the iluidizing gas in preheater 20 should be adequate to iiuidize the raw shale, i. e., to convert the same into a highly turbulent, pseudo-liquid mass of solids boiling Within the downwardly moving relatively dense mass of the heat-carrying jpebbles to form an upper interface L20. The apparent the selidstines.-

density of the fluidized shale may be about 10.-.30 pounds per cubic foot and that of the mass of pebbles about 15 45 pounds per cubic foot at linear superficial gas velocities of about 0.5-2.5 `feet per second.

The fluidized shale moves upwardly through the downwardly moving mass of heat-carrying pebbles under the inuence of the supply of shale to the bottom of preheater 20. On its way through preheater 20 the shale may be preheated to about 500 900 F. in direct countercurrent heat exchange with the heat-,carrying pebbles which may be withdrawnthrough line 23 at the temperature mentioned; above. The cooled pebbles may be returned by means of a bucket conveyor, fluid lift, or s imilar device 28 to the top of pebble h eater 75 at a rate controlled by star feeder or other metering device 29.

A suspension of preheated shale in lluidizing gas is withdrawn from an upper portion of preheater 20 via line 31 at the maximum temperature attainable in pre- .heater 20. The shale discharge may take place from a point substantially coinciding with level L20, or from a point slightly below this level, or from the dilute phase forming above level L20, in preheater 20. Suitable screening means may be provided to, prevent large heat carrier pebbles from entering line 31.

The suspension of preheated shale in fluidized gas passes through line 31 to` an intermediate portion of retort 15 at a rate controlled by valve 33. Line 31 may have the form of an aerated standpipe, a high velocity transfer line, or any other suitable fluid-solids conveying means, depending on the point of shale withdrawal from preheater 20 and the density of the suspension to be conveyed.

Retort 15 is provided in its lower portion with a gas distributing grid 35 through which a free oxygen-containing gas, such as air, oxygen-enriched ue gas, steam admixed with ilue gas and air and/or oxygen, etc., is admitted for iluidization and heat generation. This gas may be originally fed through line 37 by blower 39 and may be preheated in the space below grid 35 to about 400- 800 F. as will appear, hereinafter. The amount and composition of the gas owing through grid 35 should be so controlled that suicient oxygen is provided to generate by a limited combustion within retort 15 all the heat required for distillation` at about 800-l200"l F. and that a superficial linear lluidizing gas velocity of about 0.5-2.5 feet per second is established.

At these conditions, a dense turbulent shale bed M15 is established having an upper interface L15. The coarse and ground shale entering through lines 1,4 and 31, respectively, is rapidly heated to distillation temperature and disintegration to a particle size of about -200 microns proceeds continuously with the eect that mass M15 consists at all times predominantly of particles of this readily lluidizable size range. This disintegration may be further enhanced by providing high velocity gas jets 40 and/or 42.

A mixture of product vapors and gases, ue gases and liuidizing gas containing entrained spent shale iines is passed overhead from level L15 preferably through a gassolids separation system, such as one or more cyclone separators 4l, from which separated fines may be returned to bed M15 via dip-pipe 43. ln order to free this overhead stream completely of entrained solids, a scrubbing section consisting of two or more bubble cap plates 45 may be arranged above cyclone 41 in the top of retort l5. Heavy product oil, such as a Bunker C fuel oil fraction, may be supplied via line 47 to the top plate 45 from which it ows down over the second plate 45 and back into bed M15 via downcomers 49 to be further cracked to low boiling products in retort 15. The heat loss incurred in this scrubbing section is relatively small since only very little scrubbing oil, say about 0.005-0.l pound per pound of fresh shale, is needed to scrub out The gasiform Overhead new. vat .a `aan@rentre only about 2o 1oo F. benw distillation temperature is passed through line 13 to the bottom of heatv exchanger 7 A.to preheat the coarse shale portion therein as described above. Cooled vapors and gases containing entrained condensate are withdrawn from the tgp of .heat exchanger 7 at a temperature of about 50-250 F. and may be passed via line S1 directly to a combined settler and gasliquid separator 55. If desired, a portion or all of the contents of line 7 may be passed via line 52 to a further cooler 53 wherein light ends may be condensed with the aid of relatively minor amounts of cooling water, say about 2-l0gallons per gallon of light ends condensed. The condensate as well as uncondensed vaporsl and gases then nally enter separator 5 5. Oil is recovered from an upper liquid layer in separator 55 through line 57 and water from the bottom of separator 55 through line 59. A tail gas rich in gaseous hydrocarbons and having a heating value of about 200-800 B. t. u. per standard cubic foot is removed via liney 61 to be vented or used for any desired purpose.

Returning now to retort 15, hot spent shale is withdrawn from a lower portion of mass M15 through line 63 substantially at distillation temperature. A portion of the sensible heat of the spent shale `may be recovered in boiler 65 in the form of steam.' The partially cooled shale which may now be about 50-l00 F. below` distillation temperature is passed on through line 67 and may be split into two streams.

A minor stream of about l0-30% of the total spent shale may be passed via line 69 to the space below grid 35 in retort 15 at a point just above a bubble cap p1ate`71. The fluidizing and combustion-supporting gas supplied via line 37 to, a point below plate 71 converts this shale into a lluidized mass of hot solids serving to preheat this gas to a relatively high temperature level ot, s ay, about 400-S00 F. Spent shale of this temperature is passed lvia line 73 to an intermediate portion of pebble heater 75 by means of a suitable carrier gas supplied through line 74.

The remaining major portion of the spent shale in .l line 67 is passed via line 77 directly to a lower portion of pebble heater 75 without substantial change in temperature. Conventional iluidized solids conveying means, such as aerated standpipes, transfer lines, etc., or suitable combinations of such means (not shown) may be used to convey the spent shale to `pebble heater 75 which may be similar to preheater 20 in design and operation. A downwardly moving mass of heat-carrier pebbles is maintained in heater 75, cooled pebbles being supplied from conveyor 2S via line 79 to the top of heater 75 at a temperature of about l50300 F. A lluidizing gas, such as hot steam, air, llue gas or suitable mixtures thereof, may be supplied through line 81 and perforated distributing cone 33 to the lower portion of heater 75 at a rate sufficient to uidize the spent shale supplied through lines 73 and 77 within the mass of pebbles, substantially as described with reference to preheater 20. However, the linear superficial gas velocity in heater 75 may be somewhat lower than that in preheater 20, say about 0.1-1.5 ft. per second, because the particle size of the spent shale is normally substantially smaller than that of the raw shale fraction supplied to preheater Z0.

The uplowing hot spent shale gives off most of its heat to the downwardly moving pebbles in direct countercurrent heat exchange. A suspension of cooled spent shale in fluidizing gas may be withdrawn overhead from heater 75 either by overllow directly from the iluidized mass at level L75 via line 85 or from the upper dilute phase via line 87, or both, at a temperature of about 200-500 F. Heat-carrying pebbles may be withdrawn from the bottom of heater 75 at a temperature of about 600-l000 F. and supplied at vthis temperature to the top of--preheater 20 vialine 89 at a rate controlled by metering device 91, as described above.

When Qperating in the manner. described, Satisfatory amaaipreheat of the shale and uidizing gas is accomplished with a minimum of investment. Air consumption for heat generation may bereduced by as much as 1%, as compared to conventional operation without preheat.

The system illustrated in the drawing permits of varioils modifications. For example, both lines '73 and '77 may be operated as transfer lines and discharge directly into cone 83. Instead of generating the heat required for distillation by combustion within retort 15, any desired portion of the spent shale in line 63 may be burned in a separate, preferably fluid-type burner, and returned to retort for heat supply. Both spent shale from line 63 and burned shale from such separate burner may then be used to preheat the pebbles in heater 75 as described above.

Instead of using the spent shale on plate 71 merely as a means for preheating the combustion-supporting gas by direct heat exchange, the temperature on plate 71 may be increased to permit combustion of spent shale on this plate. For this purpose, spent shale circulation from mass Mis to plate 71 and solids entrainment into mass Mis from plate 71 are very considerably increased. This circulation rate should be high enough to maintain the spent shale above ignition temperature and simultaneously to prevent overheating of the spent shale by the combustion. Best results are obtained by maintaining the shale on plate 71 at a temperature about 50-300 F. above the desired distillation temperature. When so operating, most or all of the spent shale in line 63 may be directly passed to plate 71. In this case less, if any, combustion is required in mass M15 and less valuable product is lost by combustion. Of course, the hot spent shale in line 73 should then be supplied to a lower portion of pebble heater 75.

Other types of indirect heat exchange between spent shale and fresh shale, such as various types of shell-andtube exchangers may be used in place of the pebble heatersystem described. Other modifications within the spirit of the invention will appear to those skilled in the art.

The above description and exemplary operations have served to illustrate specific embodiments of the invention. It will be understood that the invention embraces such other variations and modifications as come within the spirit and scope thereof.

What is claimed is:

1. In the process of distilling oil shale in the form of a dense turbulent mass of subdivided solids uidized by an upwardly fiowing gasiforrn medium in a distillation zone at a distillation temperature, the improvement which comprises feeding a maior portion of said oil shale as a coarse material having a particle size of not less than about 1/2 inch in diameter to a heat exchange zone, passing a settled bed of said coarse particle size shale downwardly through said heat exchange zone, passing gasiform distillation zone effluent having a temperature not substantially below said distillation temperature upwardly through, and in direct countercurrent heat exchange with, said bed so as to preheat said coarse particle size shale and cool said effluent, and to establish a substantial temperature gradient over the height of said bed, feeding said preheated shale to said mass, withdrawing hot spent shale from said mass, preheating the remainder of said oil shale in the form of a material of uidizable size not substantially exceeding 1/1 inch in diameter by indirect heat exchange with said hot spent shale, and feeding said preheated shale of uidizable size to said mass.

2. The process of claim 1 in which said coarse shale has a particle size of about 1-3 inches in diameter and said shale of 'luidizable size has a particle size such that about -90% thereof pass through a 50 mesh screen.

3. In the process of distilling oil shale in the form of a dense turbulent mass of subdivided solids uidized by an upwardly liowing gasiform medium in a substantially vertical distillation zone at a distillation temperature, the improvement which comprises feeding a major portion of said oil shale as a coarse material having a particle size of not less than about 1/2 inch in diameter to a heat exchange zone, passing said coarse shale in the form of a settled substantially non-turbulent bed downwardly through said heat exchange zone, feeding coarse shale substantially at atmospheric temperature to an upper portion of said preheating zone, introducing gasiform distillation zone efiiuent having a temperature not substantially below said distillation temperature into a lower portion of said bed, controlling the fiow rate and residence times of said coarse shale and effluent within said preheating zone so as to preheat said coarse shale to a temperature approaching said distillation temperature and to cool said efiiuent to a temperature approaching atmospheric temperature and to establish a substantial temperature gradient over the height of said bed, withdrawing etiiuent so cooled from an upper portion of said preheating zone, feeding preheated coarse shale from a lower portion of said prcheating zone to said distillation Zone, withdrawing hot spent shale from said mass, directly and countercurrently contacting said withdrawn hot spent shale in the form of a dense, turbulent, fluidized mass of solids with a subdivided solid heat carrier having a higher gas settling rate, to heat said heat carrier, to a temperature approaching said distillation temperature, directly and countercurrently contacting said heat carrier so heated with a dense, turbulent, fluidized mass of the remaining minor portion of said shale having a tluidizable particle size not substantially exceeding M: inch in diameter to prcheat the latter, and feeding said minor portion of said shale so preheated to said distillation zone.

4. The process of claim 3 in which the flow velocity of said effluent in said preheating zone is increased on its path through said preheating zone so that said velocity reaches a level conducive to liquid entrainment at a point within said preheating zone substantially removed from the feed point of said eiuent.

5. The process of claim 3 in which said coarse shale preheating temperature is sufficiently high to reduce the mechanical strength of said coarse shale particles.

6. The process of claim 3 in which said distillation temperature is about 800-1000 F., said coarse shale preheating temperature being about 500-900 F. and said shale remainder preheating temperature being about 7. The process of claim 3 in which at least a portion of said withdrawn hot spent shale is first directly contacted with a free oxygen-containing gas to burn carbon oi said spent shale and to heat the saune and to produce a hot flue gas carrying entrained spent shale, said iiue gas and entrained shale being passed upwardly through said turbulent mass to supply heat thereto, and Spent shale so burned and heated is directly and countercurrently contacted with said heat carrier of higher gas settling rate.

References Cited in the file of this patent UNITED STATES PATENTS 1,712,082 Koppers May 7, 1929 1,712,083 Koppel-s May 7, 1929 2,285,276 Hemminger Iune 2, 1942 2,289,917 Lambrotte July 14, 1942 2,393,636 Johnson Jan. 29, 1946 2,420,376 Johansson May 13, 1947 2,449,615 Peck Sept. 21, 1948 2,626,234 Barr et al Ian. 20, 1953 2,639,263 Lefer May 19, 1953 OTHER REFERENCES Pebble Heater, Chemical and Metallurgical Engineering, July 1946,-pgs. 116-119. 

1. IN THE PROCESS OF DISTILLING OIL SHALE IN THE FORM OF A DENSE TURBULENT MASS OF SUBDIVIDED SOLIDS FLUIDIZED A DENSE TURBULENT MASS OF SUBDIVIDED SOLIDS FLUIDIZED TION ZONE AT A DISTILLATION TEMPERATURE, THE IMPROVEMENT WHICH COMPRISES FEEDING A MAJOR PORTION OF SAID OIL SHALE AS A COARSE MATERIAL HAVING A PARTICLE SIZE OF NOT LESS THAN ABOUT 1/2 INCH IN DIAMETER TO A HEAT EXCHANGE ZONE, PASSING A SETTLED BED OF SAID COARSE PATICLE SIZE SHALE DOWNWARDLY THROUGH SAID HEAT EXCHANGE ZONE, PASSING GASIFORM DISTILLATION ZONE EFFLUENT HAVING A TEMPERATURE NOT SUBSTANTIALLY BELOW SAID DISTILLATION TEMPERATURE UPWARDLY THROUGH, AND IN DIRECT COUNTERCURRENT HEAT EXCHANGE WITH, SAID BED SO AS TO PREHEAT SAID COARSE PARTICLE SIZE SHALE AND COOL SAID EFFLUENT, AND TO ESTAB- 